1. Field of the Invention
The present invention generally relates to semiconductor fabrication, and particularly to a semiconductor device, such as thin film transistor (TFT).
2. Description of Related Art
An electronic circuit is usually fabricated by semiconductor fabrication technology to form an integrated circuit. The electronic circuit usually includes a large amount of field effect transistors (FETs). The performance and the size of the transistors would also determine the performance and the size of the electronic circuit.
As to the development on design of transistor, the TFT has been proposed to as one of popular transistors. In TFT, the source electrode and the drain electrode of the TFT are formed by a semiconductor layer over the substrate instead of being formed formation in silicon substrate. The material of the semiconductor layer for forming the source electrode and the drain electrode can be metal nitride, such as the tantalum nitride (TaN).
Further, the channel used in the newly developed TFT may be formed by oxide semiconductor such as In—Ga—Zn—O (IGZO) or the similar materials, like In—Zn—O (IZO), In—Ga—O (IGO), Zn—Sn—O (ZTO), In—Sn—Zn—O (ITZO), Hf—In—Zn—O (HIZO), or Al—Zn—Sn—O (AZTO).
The source electrode and the drain electrode are formed on the oxide semiconductor, in which the channel of the TFT as a part of the oxide semiconductor is formed between the source electrode and the drain electrode. In other words, the distance between the source electrode and the drain electrode would determine the channel length.
When the channel length in design is reduced, the distance between the source electrode and the drain electrode is reduced, accordingly, and a trench with high aspect ratio would be formed. The high aspect ratio would cause the difficulty to form the gate structure subsequently, about filling into the trench between the source electrode and the drain electrode. Even further, the source electrode and drain electrode of metal nitride, such as TaN, are easily oxidized as the native oxide. The native oxide would further reduce the channel length as not intended, which may cause the malfunction of the TFT.
The design for the TFT is still at least an issue to be considered in fabricating the TFT.